What is foundation underpinning?

Underpinning is the process of installing support elements - piers, pins, or micro-piles - beneath an existing foundation footing to transfer the structure's load to deeper, more stable soil or bedrock. It is used when the soil at the original footing depth has lost bearing capacity through settling, shrinkage, or erosion.

What types of piers are used for underpinning?

The three most common systems are: helical piers (screwed into the ground, suitable for most conditions), push piers/resistance piers (hydraulically driven using the structure's weight), and concrete piers (drilled and poured on-site). Each has different appropriate use cases based on soil type, load, and access.

How many piers does a typical underpinning project require?

Most residential projects require 6-15 piers. The number depends on the load distribution, the extent of settling, and the spacing recommended by engineering standards (typically one pier per 6-8 feet of affected perimeter). Interior piers are added when the center of the slab is also settling.

Does underpinning work permanently?

Yes, when properly installed to stable bearing depth. The piers transfer load to soil or rock that is not subject to the same movement causing the original settlement. Most reputable contractors offer 25-year to lifetime transferable warranties on pier work.

Can underpinning lift my foundation back to original grade?

Often, partially. Contractors can lift settling areas by pressurizing piers to lift the structure. However, over-aggressive lifting can crack plumbing connections and interior finishes. Most contractors target a 'practical lift' - reducing differential movement - rather than attempting to restore original elevation in full.

Foundation Underpinning | Helical Piers, Push Piers & Concrete Piers

When Is Underpinning Necessary?

Underpinning is the right solution when:

The foundation has settled differentially (one part lower than another)
The settlement is caused by inadequate soil bearing capacity at footing depth
The soil at footing depth will not recover bearing capacity on its own (e.g., consolidating clay, loose fill)
The structure requires stabilization before other repairs (cosmetic, plumbing) will hold

Underpinning is not the right solution for:

Heaving foundations (soil swelling upward) - these require drainage correction, not piering
Cracks caused by thermal movement or concrete shrinkage - these are cosmetic issues
Settlement caused solely by a plumbing leak - fix the leak first, then reassess

Underpinning Methods Compared

Method	Mechanism	Best For	Depth	Cost/Pier
Helical piers	Screwed in with torque	Most conditions; lighter loads; interior use	15-30 ft	$1,200-$3,500
Push/resistance piers	Hydraulically driven	Perimeter of heavy structures	20-40 ft	$1,000-$3,000
Drilled concrete piers	Drilled and poured on-site	Deep unstable soils; high loads	15-40 ft	$800-$2,500
Micro-piles	Drilled small-diameter piers	Tight access; rocky soil	20-50 ft	$1,500-$4,000

Helical Piers

Helical piers are steel shafts with helical bearing plates welded at intervals. A hydraulic motor rotates them into the ground - essentially screwing them in. Their installation torque correlates predictably with bearing capacity, allowing engineers to calculate when adequate depth is reached.

Advantages: Work in almost all soil types; suitable for interior use; lower vibration than driven piers; can be installed in tight clearances.

Limitations: More expensive per pier than push piers in some cases; not appropriate for very shallow stable rock.

Push Piers (Resistance Piers)

Push piers are driven hydraulically using the dead load of the structure itself as reaction force. Pier sections are added and driven progressively until refusal depth (stable bearing) is reached. The same hydraulic equipment then lifts the structure.

Advantages: Typically less expensive than helical piers on the perimeter; reach substantial depth; the bearing test is direct (you’re loading the structure).

Limitations: Require adequate dead load on the footing - not suitable for lighter structures or interior use without modification; more disruptive to the perimeter.

Concrete Bell-Bottom Piers

Common in Texas, drilled concrete piers are bored with an auger to a prescribed depth, then a bell-shaped base is formed at the bottom (in appropriate soil). Concrete is poured in the bore hole. These are typically engineered and permitted.

Advantages: Proven technology in expansive clay regions; can be designed with precise bearing capacity; often the local standard in Texas markets.

Limitations: Cannot be installed in all soil types; require adequate cure time before loading; waste disposal from the bore hole.

The Underpinning Process

Engineering review - Some contractors include a basic engineering assessment; for larger projects, an independent structural engineer report is recommended and may be required for permit.
Access preparation - Piers are installed from the exterior perimeter (via trench) or interior (via core holes in the slab). Perimeter trenching is typically 18-24 inches deep.
Pier installation - Piers are installed at designed spacing. For push piers, each pier is driven to refusal; for helical piers, driven to minimum installation torque.
Lifting - Hydraulic jacks at each pier position lift the structure simultaneously. A laser level monitors grade during lifting.
Bracket and grouting - Brackets lock the load transfer. Core holes are patched. Trenches are backfilled.

What Underpinning Will Not Fix

Underpinning stabilizes movement - it doesn’t repair the cosmetic damage caused by that movement. Interior drywall cracks, stucco cracks, tile damage, and misaligned doors and windows require separate cosmetic repair after the foundation is stable.

Plumbing damage from foundation movement (sheared slab lines) is also a separate repair that must be addressed after stabilization.

Foundation Underpinning: How It Works and When You Need It